1. Field of the Invention
The invention relates generally to hydraulically driven pump jacks and, more particularly, but not by way of limitation, it relates to an improved form of servo-controlled hydraulic drive system as may be utilized on an oil well pumping unit.
2. Description of the Prior Art
The prior art includes numerous types of drive systems for use in actuation of walking beam types of pumping units. Earlier types employed a fuel-driven engine and crank linkage to move the walking beam in an up/down pumping motion, and it has been attempted to use direct electric motor drive but with much loss in power efficiency. More recently, a number of systems have evolved which utilize hydraulic power to reciprocate the walking beam. In general, these units use a valving system which alternates from full open to closed, and many problems are encountered due to the fact that drive power is not controlled in accordance with the sine function of pump head movement. U.S. Pat. No. 4,201,115 teaches an oil well pump jack with dual hydraulically operated piston and cylinder assemblies controlling movement of the walking beam. This patent teaches the use of a mechanical linkage from the walking beam to provide servo input through a reversing valve thereby to effect reciprocal energization to drive the walking beam.
U.S. Pat. No. 3,939,656 teaches yet another form of servo-feedback structure in order to provide reversing fluid circulation to a double-acting hydraulic cylinder which drives the walking beam. The servo system of this teaching requires additional hydraulic circuitry and double-acting control cylinder in order to effect synchronous reciprocation as controlled by a variable displacement reversing swashplate pump that is driven at a predetermined constant speed. Finally, U.S. Pat. No. 3,175,513 deserves mention in that it discloses a hydraulic pumping unit that has a reciprocal rate control means. A sliding weight on the walking beam is controlled by a hydraulic cylinder in order to balance the rate of the walking beam optimally during its rocking action.